What is a drug allergy?

A drug allergy happens when you have a harmful reaction to a medicine you use. Your body’s immune system fights back by setting off an allergic reaction. Most drug allergies are mild, and the symptoms go away within a few days after you stop using the medicine. But some drug allergies can be very serious.

Some drug allergies go away with time. But after you have an allergic reaction to a drug, you will probably always be allergic to that drug. You can also be allergic to other drugs that are like it.

A drug allergy is one type of harmful, or adverse, drug reaction. Symptoms and treatments of different kinds of adverse reactions vary. So your doctor will want to find out if you have a true drug allergy or if you have another type of bad reaction that isn't as serious.

What are the symptoms?

The symptoms of a drug allergy can range from mild to very serious. They include:

• Hives or welts, a rash, blisters, or a skin problem called eczema. These are the most common symptoms of drug allergies. See a picture of skin reactions caused by drug allergies.
• Coughing, wheezing, a runny nose, and trouble breathing.
• A fever.
• A serious skin condition that makes your skin blister and peel. This problem is called toxic epidermal necrolysis, and it can be deadly if it is not treated.
• Anaphylaxis, which is the most dangerous reaction. It can be deadly, and you will need emergency treatment. Symptoms, such as hives and trouble breathing, usually appear within 1 hour after you take the medicine. Without quick care, you could go into shock.

What medicines commonly cause an allergic reaction?

Any medicine can cause an allergic reaction. A few of the most common culprits are:

• Penicillins (such as nafcillin, ampicillin, or amoxicillin). These types of medicines cause the most drug allergies.
• Sulfa medicines.
• Barbiturates.
• Insulin.
• Vaccines.
• Anticonvulsants.
• Medicines for hyperthyroidism.

If you are allergic to one medicine, you may be allergic to others like it. For example, if you are allergic to penicillin, you may also be allergic to similar medicines such as cephalosporins (cephalexin or cefuroxime, for example).

People with AIDS or lupus may be allergic to many types of medicines. The reactions usually aren't dangerous, but they can make it hard to treat the disease.

Some people-especially those with asthma-have reactions to common pain relievers such as aspirin and ibuprofen. These seem like allergic reactions but they are not, because they do not affect the immune system. But these reactions can be severe in people who have asthma.

How is a drug allergy diagnosed?

Your doctor will diagnose a drug allergy by asking you questions about the medicines you take and about any medicines you have taken in the recent past. Your doctor will also ask about your past health and your symptoms. He or she will do a physical exam.